High voltage transformers, such as are used for transforming voltages such as 38 kV to 110 volts for use in medium voltage switchgear metering and relaying applications, must be grounded when they are removed from the power source because such devices are capable of storing a substantial charge on the transformer winding. This charge can injure a repair person who inadvertently touches a disconnected transformer terminal if the charge has not been removed from the transformer. Thus, it has become a requirement that voltage transformers be grounded as they are removed from the power source. Typically, this grounding is achieved by a momentary brushing of grounding contacts mounted on the device drawer such that when the drawer is opened, or the device is pulled out, the grounding contacts make momentary contact with the system ground, thereby discharging the transformer.
Several problems can occur using such a method. One major obvious problem is the simple fact that the contacts may not touch because of misalignment or because they are broken. Thus, a technician who makes the erroneous assumption that the contacts have discharged the transformer is placed in jeopardy of serious, if not fatal, injury.
An additional problem with existing systems is that the grounding contacts momentarily mate only after the drawer or drawout element has been almost fully removed from the cabinet. Thus, if the drawer has not been fully opened, or the drawout not fully removed, the transformer remains charged and a technician could reach in and touch the energized transformer or associated equipment.
Another problem with existing grounding systems is that typically the actual circuit ground to the power source is broken soon after the transformer is momentarily grounded and therefore the entire cabinet becomes electrically isolated. This could present problems if, for example, a flashover occurs from the power source to the pull-out drawer or drawout element, again resulting in potential injury to an operator.
Accordingly, it is one desire to construct a transformer removal system such that an operator can visually determine that the transformers have been grounded so that if such grounding has not occurred with respect to a particular transformer, the operator can visually make such a determination.
It is a further desire to design a power system such that when a device, such as a transformer, is withdrawn from the power source, there is a permanent ground on the terminals so that the device cannot become inadvertently reconnected to a power source.
It is a still further desire to create a removable transformer system such that the main chassis ground is maintained throughout the entire travel of the drawout element/drawer or at least until the device is moved a substantial distance from the power source.